Angiotensin II, when injected into spiny dogfish sharks, increases the arterial blood pressure because it causes the release of epinephrine from special storage places called chromaffin cells. Angiotensin II also increases arterial blood pressure in mammals, including man, and its role in the control of normal pressure and as a possible cause of hypertension is under investigation. This study will attempt to find out if angiotensin II acts directly on the chromaffin cells to release epinephrine or if it first stimulates some part of the brain which then transmits nerve impulses to the chromaffin cells and effects the release of epinephrine. This primitive species does not produce angiotensin endogenously but exhibits the same epinephrine release response which is also seen in mammals. Thus, experiments on the dogfish shark provide an opportunity to study a very primitive response to a substance which may be implicated in hypertensive disease in man.